1. Field of the Invention
The present invention relates to a balancer for the main spindle unit of a machine tool, especially, a balancer that makes it easy to replace annular sealing members of a gas spring provided for supporting the main spindle unit to reduce the load of an elevating mechanism that drives the main spindle unit up and down.
2. Description of the Related Art
A machine tool such as a machining center generally has a main spindle unit that has a main spindle on which a tool is mounted and an electric motor that drives the main spindle rotationally, wherein such a main spindle unit is supported and moved up and down by means of an elevating mechanism. This kind of elevating mechanism is normally equipped with a vertically arranged ball screw shaft, a ball screw nut that engages with said ball screw and is connected to said main spindle unit, and an electric motor, wherein the main spindle unit moves up and down together with the ball screw nut as the ball screw shaft is turned by the electric motor.
However, it is necessary to move a heavy main spindle unit up and down by means of an elevating mechanism in this kind of machine tool, so that the load of the elevating mechanism is heavy, consequently affecting the positioning accuracy of the main spindle unit in the vertical direction and making it difficult to increase the speed of the vertical motion of the main spindle unit. Moreover, such an elevating mechanism that supports a heavy main spindle unit tends to be large in size and is expensive to manufacture.
The applicant of the present application has developed a system that uses a gas spring as a main spindle head balancer and succeeded in making it practically useful. This main spindle unit balancer generates an upward thrust that substantially equal to the weight of the main spindle unit by means of the gas spring in order to reduce the load applying on the elevating mechanism.
As shown in FIG. 6, a gas spring 101 of this main spindle unit balancer 100 comprises a cylinder body 102 filled with a compressed gas G and a rod 103 that extends downward through a rod-side end wall member 105 of this cylinder body 102. A head side end wall member 104 of cylinder body 102 is provided with a filling valve 106 for filling the compressed gas G into cylinder body 102. A nut member 107 is provided on the upper end of rod 103, being screwed onto it and preventing it from slipping away from rod-side end wall member 105. The height of cylinder body 102 is substantially equal to the stroke of rod 103.
In order to prevent the compressed gas G in cylinder body 102 from leaking and gradually losing the gas pressure, rod-side end wall member 105 is provided with a pair of annular sealing members 108 and 109 sealing between rod-side end wall member 105 and rod 103. Annular sealing members 108 and 109 have to be replaced periodically as they wear out due to frictional sliding of rod 103.
However, while it is necessary to replace annular sealing members 108 and 109 periodically in said main spindle head balancer 100, the replacement also requires gas spring 101 itself to be removed from the main spindle unit, which necessitates in turn a space beneath the main spindle for the removal. This presents some restrictions on the design of the main spindle unit in that the location of gas spring 101 has a certain limitation and that a consideration from the standpoint of maintenance work is required in determining the stroke of the main spindle unit. Moreover, the replacement work is more time-consuming.
The purpose of the present invention is to provide a balancer for the machine tool main spindle unit that makes it possible to easily replace the annular sealing members without having to remove the gas spring.